Calibration standard for 2-D and 3-D profilometry in the sub-nanometer range and method of producing it

ABSTRACT

A calibration standard comprises a supporting structure (1) of single crystal material with at least one pair of different kinds of structures consisting of a raised line (2) and a trench (3). These structures have the identical width in the range of about 500 nm. The single crystal material preferably is silicon with (110)-orientation. A method of producing the calibration standard comprises the steps: providing two polished wafers of the same single crystal material and with the same crystal orientation, forming an oxide layer on the polished surface of the first wafer, bonding the second wafer to the first oxidized wafer with the polished surfaces of the wafers facing each other, cutting the bonded structure transverse to the polished surfaces, selectively etching both the wafers to a defined depth to expose a portion of the oxide layer, masking the portions of the oxide layer now representing the raised line (2) and selectively etching the oxide layer in the unmasked areas to a defined depth to form the trench (3). The calibration standard overcomes the problem of measuring the diameter of an ultrafine tip for AFM/STM profilometry in the sub-nanometer range.

FIELD OF THE INVENTION

The present invention relates to a calibration standard for theprofilometry in the sub-nanometer range, especially for the Atomic ForceMicroscope/Scanning Tunneling Microscope (AFM/STM) profilometry and to amethod of producing said calibration standard.

BACKGROUND OF THE INVENTION

Nanometrology stands for the measurement technology necessary toinvestigate, to develop and to verify the measuring features ofinstruments and of materials standards with an uncertainty best given inunits of nanometers. For the calibration of different probe systems anddifferent measuring instruments reference standards are asked for.

2-D AFM profilometry uses specially shaped ultrafine silicon tips with adiameter of typically about 250 nm. The absolute measuring precisenesssolely depends on the exact knowledge of the tip diameter which usuallyis determined by a high resolution SEM with a measuring accuracy of 3 to5%.

With a tip diameter of 250 nm this leads to a measuring precisenessworse than about 7.5 nm. Within the electron microscope, depending onthe rest gas concentration, the tip is more or less contaminated andthus its diameter is changed indefinitely during measuring. To improvethe measuring accuracy the tip should be calibrated directly before andafter the measurement with a gauge.

Other common solutions also focus on the application of single-crystalsilicon technology.

A standard reference material (SRM) consisting of a silicon wafer with asilicon dioxide film of uniform thickness is described in SPIE Vol. 661,402 (1986) "Film thickness and refractive index Standard ReferenceMaterial calibrated by ellipsometry and profilometry" by G. A. Candelaet.al. The silicon dioxide film contains windows used for stylusprofilometry measurements with a mechanical depth very nearly the sameas the oxide thickness. The depth is not exactly the same as the oxidethickness due to native oxide forming in the window with an averagethickness of about 2 nm. This affects the accuracy of the measurement.

In Metrologia, 1991/92, Vol.28, pp. 443-453, "Nanometrology at the PTB"by H. Kunzmann, a reference scale in the sub-nanometer range is derivedfrom a silicon single crystal epitaxially grown by chemical gastransfer. With high probability the surface of this crystal is planewithin one lattice plane and it is proposed to use the steps which aresmall-integer multiples of lattice plane distances for the calibrationof probe systems for nanometrology. These steps are results of theepitaxial processes and they could be used to manufacture step-heightgauges, but only if the technology of epitaxial growth could be broughtunder well-defined metrological control. Although realizing accuraciesin the sub-nanometer range, this solution only covers 1-D displacementmetrology.

SUMMARY OF THE INVENTION

In accordance with the present invention, a calibration standard andmethod for calibrating is described comprising a supporting structure ofsingle crystal material with at lease one pair of different kinds ofstructures, the structures including at least a trench and a raised linehaving substantially the same width.

The invention further provides an array of structures wherein the widthsare different between selected structures in said array.

It is therefore an object of the invention to provide a calibrationstandard for 2-D and 3-D profilometry with an accuracy in the range of 1nm and better.

The invention also comprises methods of producing a calibration standardand the use of the standard for measuring features in the sub-nanometerrange.

BRIEF DESCRIPTION OF THE DRAWING

These and other features, objects, and advantages of the presentinvention will become apparent upon consideration of the followingdetailed description of the invention when read in conjunction with thedrawing in which:

FIG. 1. shows the calibration standard with one pair of different kindsof structures.

FIG. 2a shows an array of pairs.

FIG. 2b shows an array of dies on a wafer.

FIG. 3 shows the calibration standard with one pair of different kindsof structures and the measured profile lines.

FIGS. 4A-4E show cross-section views of the process steps of a preferredmethod of producing the calibration standard.

FIGS. 5A and 5B show cross-section views of a preferred method ofmeasuring features like ultrafine silicon tips with the calibrationstandard.

FIGS. 6A-6D show some results of measurements made with an ultrafine tipand the calibration standard.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, FIG. 1 shows the basic structure of thecalibration standard consisting of a supporting structure 1 and one pair4 of different kinds of structures 2, 3.

The first kind of structure is a raised line, tablet or plate 2 and thesecond kind of structure is a trench or opening 3. Both of thestructures, the raised line, tablet or plate and the trench or opening,are of exactly the same width.

Since this basic structure is used as a calibration standard, the widthof these structures 2,3 should be comparable to the lateral and verticaldimensions of the features to be measured. This has the advantage thatthe calibration as well as the measuring step may be carried out withinthe same operating area of AFM/STM profilometry.

Nowadays, the smallest lateral dimensions to be measured are thesmallest (metal) linewidths of the semiconductor chips, typically in therange of 500 nm (16 Mbit-chip) or of 350 nm (64 Mbit-chip). Typicalvertical dimensions are smaller than approximately 2000 nm.

The width of the trench and the raised conductive line as well as theirvertical dimension therefore preferably lie in the range from 350 to2000nm and may be, for example, approximately 500 nm.

In a preferred embodiment, groups 6 of basic structures or pairs 4 ofdifferent kinds of structures 2, 3 of a group 6 having the same widthare provided as shown in FIG. 2A. Groups 6 of basic structures or pairs4 form an array 5 and several arrays 5 of calibration structures may bearranged on a substrate 7, preferably of silicon material.

In the embodiment shown in FIG. 2B, array 5 of groups 6 of basicstructures with different line widths are arranged on a relatively smallarea of a few square millimeters on a silicon wafer, typically in a dieof about 1 cm². For calibrating different tips or for calibratingpiezoelectric actuators this is very important since it is required tohave a calibration standard with structures 2, 3 showing different linewidths.

One method to produce a calibration standard as described above startswith providing two polished wafers 8,9 as shown in FIG. 3. These wafers8, 9 have the same single crystal material with the micro roughness ofthe polished surfaces 11, 12 respectively being better than 1 nm. Thepreferred material is silicon. Both wafers 8, 9 have the same crystalorientation with respect to its surface 11, 12. For the method describedhereinafter their surface 11, 12 is a (110)-surface. On the polishedsurface 11 of the first wafer 8 an oxide 10 is grown by thermallyoxidizing the wafer 8 until an oxide thickness corresponding to thewidth of the raised line 2 and the trench 3, which are to be formedlater, is reached. The thickness of the oxide layer 10 thus lies in therange from 350 to 2000 and may be, for example, approximately 500 nm. Ina next step the second wafer 9 is bonded to the oxide layer 10 grown onthe first wafer 8. This is done in a way that the polished surfaces 11,12 of both the wafers 8, 9 are facing each other.

In the following steps, especially when the structures 2, 3 of thecalibration standard will be formed, this detail becomes important sincethe polished surfaces 11, 12 will form the walls of the trench 3. Thebonding step typically is fusion bonding or anodic bonding of the twowafers 8, 9 and produces a chemical interconnection between the twowafers at the interface 12.

The bonded sandwich structure thus achieved is then cut transverse, forexample, orthogonal (rectangular) to the polished surfaces 11, 12 of thebonded wafers 8, 9 by a sawing step and the surface of cut 13 ispolished and cleaned in a known manner.

Breaking of the sandwich structure is not appropriate since this oftencauses unwanted micro cracks at the interface 12.

In this preferred embodiment the surface of cut 13 is a (111)-surfacedue to the use of wafers 8, 9 with (110)-wafer surfaces.

By embedding the silicon dioxide layer 10 between the polished surfaces11, 12 of the wafers, damage of the silicon dioxide layer 10 is avoidedduring polishing. By depositing a thin film onto the oxide layer 10instead of bonding a wafer 9, subsequent damage of the oxide film 10would occur when the surface of cut 13 is polished.

As a next step, the wafer material 8, 9 embedding the oxide layer 10 isselectively etched back from cut 13 to a defined depth of about 500 nm.To avoid sidewall contamination of the raised line 2 to be formed and toachieve a high selectivity of at best 1000:1 between the wafer material,preferably a semiconductor such as silicon, and the oxide layer 10,silicon dioxide, anisotropical wet etching is chosen instead of plasmaetching. Isotropical wet etch solutions normally show poorer selectivityof about a factor 10.

With a cesium hydroxide anisotropical wet etch solution a selectivity ofabout 800:1 may be expected. Although when etching with the desiredselectivity of 1000:1 the upper edges of the raised line 2 will beshrunk of approximately 0.5 nm. This effect, however, is reproducibleand does not affect the use of the calibration standard for measuringfeatures.

Now the calibration standard shows a raised silicon dioxide line 2 overits entire length.

Next, portions of the raised line 2 are masked with an etch protectinglayer of, for example, silicon nitride. In the unmasked areas the oxidelayer 10 is etched to a defined depth of about 500 nm in a wet etchsolution. A suitable etch solution is buffered hydro fluoric (BHF) acidshowing an extremely high etch selectivity between the silicon dioxidelayer 10 and the silicon of the embedding wafers 8, 9.

The trench 3 thus produced has sidewalls being ideally planar andparallel to one another with a micro roughness of less than 1 nm.

After etching to form trench 3, native oxide will grow on the siliconsidewalls of trench 3 thus narrowing it by approximately 1 nm. Sincethis narrowing is constant along the whole length of trench 3 and sinceit is reproducible, it does not disturb the measuring of features withthe calibration standard.

Another possibility of producing the calibration standard is shown withits different process steps in FIGS. 4A to 4E.

The base material is a one-side polished wafer 7' of single crystalmaterial with a (110) orientation of its surface, preferably silicon. InFIG. 4A a layer 15 of silicon dioxide is deposited on surface 21 ofwafer 7' and a pattern is exposed in a photoresist and transferred bydry or wet etching into the silicon dioxide layer 15. The edges of thepattern have to be aligned extremely precise parallel to (111) planes ofthe wafer 7. The pattern is anisotropically etched into surface 21 ofthe wafer by preferably KOH. The pattern comprises a pattern of trencheswith different widths with the trenches being arranged in several groupsand arrays.

The trenches 14 thus created are confined by (111) planes with two ofthe (111) planes being parallel and vertical with respect to surface 21of wafer 7'.

After removal of the residual silicon dioxide mask, the trenches 14 arefilled by material 16 which exhibits extremely high etchingselectivities compared to the single crystal material. Materials whichcan be used are thermal or chemical vapor deposited silicon dioxide andsilicon nitride.

The material 15 on top of the wafer 7' is then etched, polished orchemically and mechanically polished back to the original wafer surface21 as to be seen in FIG. 4C.

The single crystal wafer material 7' is then selectively etched backagainst the trench filling material 16 to a defined depth as shown inFIG. 4D. Now a masking process is used to protect a portion or half ofeach row of rectangular structures 2 with a masking material. In theother unprotected portion or half, the trench filling material 16 isselectively etched back against the single crystal material 7' to adefined depth to create trenches 3 as may be seen in FIG. 4E.

The masking material is selectively removed and the wafer 7' is dicedinto dies, each die containing now a multitude of calibration structuresof different widths as shown in FIGS. 2A and 2B.

In FIGS. 5A and 5B, there is shown a preferred method of measuringfeatures in the sub-nanometer range such as ultrafine silicon tip 17with the calibration standard. Silicon tip 17 may have cylindrical andconic portions to provide circular cross sections when taken along the zaxis.

To determine the width of an ultrafine tip 17 only two measurements arecarried out. First the width of a trench 3 is measured by profiling thetrench 3 with the tip 17 along a path shown as dashed line 19 in FIG. 3providing a value b1.

Second the width of a raised line 2 of the same pair 4 of structures 2,3 is measured by profiling the raised line 2 with the same tip 17 alonga path shown as dashed line 18 in FIG. 3 providing a value b2.

To calculate the exact diameter or width of tip 17, the measured valuesb1 and b2 are subtracted from each other and the resulting value isdivided into equal halves according to the formula ##EQU1## with t beingthe diameter of the tip 17.

The lateral dimension d of the raised line 2 and the trench 3 shown inFIG. 3 then is given by ##EQU2##

This shows that the knowledge of the exact dimensions of the pair 4 ofstructures 2, 3 of the calibration standard is not necessary todetermine the width of a feature like an ultrafine tip 17 to bemeasured. It is very important that both of the structures 2, 3, theraised line and the trench, show exactly the identical width and the twomeasurements needed are carried out with the different structures of thesame pair of structures to assure the accuracy of the calibration.

The accuracy strongly depends on the differences in the lateraldimensions, the parallelity of the sidewalls as well as on the microroughness of the sidewalls between the two different kinds of structures2, 3.

Therefore, it is important to find optimized production processes toassure that the afforded quality of the parameters is met. Suitableproduction methods are those described above.

Some results of measurements made with an ultrafine tip and thecalibration standard are shown in FIGS. 6A-6D proving the accuratenessof this measurement technique. FIGS. 6A-6D shows the repeatability ofmeasurements made in a 716 nm wide trench. In FIGS. 6A-6B the ordinaterepresents distance in nanometers along the z axis shown in FIG. 3 andthe abscissa represents distance in nanometers along the x axis shown inFIG. 3. In FIGS. 6A-6D dotted curves 31-34 show the actual measurementsmade with an ultrafine tip. Reference line 35 is above the bottom oftrench 3 shown by line 37 for measuring trench width at the bottom.Reference line 39 is below the top of trench 3 shown by line 41 formeasuring the trench width at the top. Lines 43 and 45 represent theside walls of trench 3.

These measurements demonstrate a repeatability of about 1 nm.

While there has been described and illustrated a calibration standardfor ultra fine tips for AFM/STM profilometry comprising a raised lineand a trench of substantially the same width and the method making suchstructures, it will be apparent to those skilled in the art thatmodifications and variations are possible without deviating from thebroad scope of the invention which shall be limited solely by the scopeof the claims appended hereto.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent is:
 1. A calibration standard comprising:asupporting structure (1) of single crystal material with at least onepair (4) of different kinds of structures (2,3) wherein said at leastone pair has one of said different kinds of structures extending above amajor surface of said supporting structure, the structures (2,3) of eachof said pair (4) of different kinds of structures having substantiallythe same width and sidewall surfaces exposed for profilometry.
 2. Thecalibration standard of claim 1 wherein said at least one pair (4) ofdifferent kinds of structures (2, 3) comprises an array (5) of saidpairs.
 3. The calibration standard of claim 2 wherein said differentkinds of structures (2, 3) of groups (6) of said pairs (4) forming saidarray have different widths.
 4. The calibration standard of claim 1wherein said different kinds of structures (2, 3) comprise a raised line(2) and a trench (3).
 5. The calibration standard of claim 4 whereinsaid single crystal material of said supporting structure (1) comprisesa semiconductor and said trench (3) has a portion filled with materialselected from the group consisting of silicon dioxide and siliconnitride and said raised line (2) is made of said same selected material.6. The calibration standard of claim 1 wherein said single crystalmaterial has a (110)-orientation and said width of said different kindsof structures (2, 3) is in the range from 350 to 2000 nm.
 7. Thecalibration standard of claim 4 wherein said width of said raised lineand said trench are identical.
 8. The calibration standard of claim 5wherein said semiconductor is silicon.